compass surveying lecture notes-2

 

1.2  Procedure of taking observations with Prismatic Compass

1.      The prismatic compass is mounted on the tripod such that it is exactly over the ground station.

2.      A small stone is dropped from the bottom centre of the compass. If it falls on the peg driven on the ground, the centre point is correct.

3.      The surveyor has to judge whether the compass is in Level or not by sighting only. If not, it is levelled by ball and socket arrangement.

4.      Turn up the prism and the sighting vane vertically. Raise or lower the prism until the gradations on the ring are visible.

5.      Then turn the compass box until the ranging rod at the station is bisected by the hair when sighted through the slit above the prism.

6.      When the needle comes to rest look through the prism and note the reading at which the hairline produced appears to cut image of the graduated ring. Thus, the required bearing of the line is found.

1.3  Surveyor’s compass

1.      In the surveyor’s compass the gradation on the horizontal circle is in the quadrantal system. The value of the bearings thus varies from 0^o to 90^o.

2.      The north and south are marked 0^o. east and west are marked 90^o.

3.      Prism is not provided in this compass.

4.      The surveyor’s compass has now become obsolete.

 

1.4  Designation of bearings

1.4.1        Whole Circle Bearing System (W.C.B system)

In this system, the bearing of a line is always measured clockwise from magnetic north. The angle thus measured is the Whole circle Bearing of the line. Its value ranges from 0^o to 360^o.

 

1.4.2        Quadrantal Bearing System (Q.B system)

In this system, the bearings of lines are measured clockwise or anticlockwise from north or south, whichever is nearer to the line.

  

 

 

 

1.4.3        Reduced bearing (R.B)

When the W.C.B of a line exceeds 90^o, it must be reduced to the corresponding angle less than 90^o and the value recorded along with the quadrant in which its value falls. This angle is known as the reduced bearing.

Case	WCB between	RB	Quadrant
1	0o-90o	W.C. B	NE
2	90O-180O	180o-W.C.B	SE
3	180O-270O	W.C.B-180o	SW
4	270O-360O	360o-W.C.B	NW
The above table shows conversion of bearings from W.C.B system to Q.B system

 

The below table shows conversion of bearings from Q.B system to W.C.B system
Case	Q.B system	W.C.B system
1	NE	Q.B
2	SE	180-SE
3	SW	180+SW
4	NW	360-NW

 

1.5  Fore bearing and Back bearing

1.5.1        Fore bearing (F.B)

The bearing of a line in the direction of progress of the survey is called the Fore Bearing.

1.5.2        Back bearing (B.B)

The bearing of a line in the opposite direction of progress of the survey is known as Back bearing.